Differential Settlement Causes Unstable Foundations

Poor Soil Preparation and Drainage are the Root Causes for Differential Settlement of Building Structures

By Mark J. Donovan

If you observe certain types of cracks in your home’s foundation, or even in the main home structure itself, it could be caused by differential settlement of the home. Differential settlement is the uneven downward movement of a home’s foundation.

It is typically caused by varying soil conditions underneath the home. When the home settles unevenly different parts of the home’s foundation can sink or rise at different rates. When this happens, cracks in the foundation and the main home structure can occur.

If not resolved the home could have severe operational and structural problems that could render the home unsafe. It can also make the home un-saleable.

One telltale sign that your home is experiencing differential settlement is a concentration of cracks in the corners of door frames, window frames, rooms, and wherever structural members meet within walls.

Poor water drainage under and around the home can also exacerbate differential settlement. If for example the home was constructed on top of sandy soil, the sand can erode where water travels through it. This can result in voids, or mini sink holes, underneath the home’s foundation.

Eventually the home’s foundation sinks in this area, causing pressure and stress issues in the rest of the home’s foundation and building structure. 

Differential Settlement Causes Unstable Foundations

The excess pressures eventually cause stress relief cracks in the concrete foundation walls and the building structure itself.

Similarly, homes built on clay type soil are susceptible to differential settlement. The clay can swell and contract relative to the amount of water that is flowing in or near the clay soil. Typically there is more water near the extremities of the house foundation. Consequently the clay can swell near the perimeter of the foundation faster than in the middle of the home’s foundation. This causes the edges of the foundation to rise up more quickly than the rest of the home. Thus, pressure cracks in the foundation or cement slab occur.

Key to avoiding a differential settlement issue with your home is to have a properly designed foundation installed. Part of the design includes taking soil samples and then having a soil expert / structural engineer determine the best course for preparing the soil for the installation of a foundation on it.

Similarly, if your home is experiencing differential settlement it is best to call in a structural engineering and a foundation installer to assess the situation, including the soils, to determine the best course of action to resolve the problem. It is important to note, however, that resolving a differential stress issue is expensive.

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Doing nothing, however, can be more expensive, and dangerous. So if you see a number of stress cracks in your home’s foundation walls or in the building structure don’t wait. The sooner you resolve the problem the less money you’ll spend making repairs to your home.

For more help on building a home addition, see HomeAdditionPlus.com’s Home Addition Bid Sheets. Our Home Addition Bid Sheets provide you with the knowledge and information on how to plan a home building project, and what to look for when hiring contractors. They also include detailed cost breakdown tables and spreadsheets for estimating your own new home construction building costs.

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